Why are gravitons expected to exist when gravity is just warped spacetime?

[thegroundhog]

TL;DR Summary

Why are gravitons expected to exist when gravity is just warped space time?

As per the summary I don't understand why physicists talk as if gravitons are inevitable, when gravity is just curved spacetime? Why would curved spacetime have a particle?

 

[PeroK]

There's currently no good explanation of how spacetime curvature arises from elementary particles. It's expected that this must be explicable through a quantum theory of gravity, with the graviton as the mediator.

 

[Ibix]

I don't think physicists do talk as if gravitons were inevitable. Quantum gravity, yes, but I don't think all current candidates for a theory of quantum gravity include gravitons, although I might be wrong.

Quick answer: sources of gravity can all be described in terms of quantum fields, with all the attendant stuff like superposition. So if you have a source in a superposition of states presumably you have its gravitational field in a superposed state (the source wouldn't be superposed if you could determine its state by looking at its gravitational field). That means we need a theory of quantum gravity. We just don't know how to do it yet.

 

[ohwilleke]

Summary:: Why are gravitons expected to exist when gravity is just warped space time?

As per the summary I don't understand why physicists talk as if gravitons are inevitable, when gravity is just curved spacetime? Why would curved spacetime have a particle?

It isn't so much that they are inevitable. It is that a theory of spin-2 massless gravitons with a coupling that is a function of the mass-energy of another particle and a coupling constant functionally related to Newton's constant is essentially identical in its behavior as a theory of gravity to General Relativity in the classical limit. They wouldn't be identical, because quantum gravity would have some properties that are exclusive to quantum theories that are absent in General Relativity (e.g. quantum tunneling and the stochastic nature of the theory). They would only be identical in circumstances where those quantum theory specific properties are negligible in importance. But, the vast majority of situations where we use General Relativity are situations where the classical limit of a quantum gravity theory and General Relativity are indistinguishable with current technology.

There are deep theoretical inconsistencies between the Standard Model of Particle Physics and General Relativity that have a lot to do with the Standard Model being formulated as a quantum theory in Minkowski space (where special relativity, but not general relativity applies), while General Relativity is formulated as a classical, deterministic theory in space-time that has geometric curvature.

Naively, a graviton based theory seems like a good way to reconcile these inconsistencies (not the only way, but certainly the most obvious one). But, it turns out that making a quantum gravity theory along these lines that works is hard (among other things the most straightforward approach to doing this is "non-renormalizable" which from a practical perspective means that it is basically impossible to do lots of kinds of calculations with a theory like this in our current state of knowledge).

In sum, if there is such a thing as quantum gravity, we know lots of qualitative things about what the theory would look like and especially, what properties a graviton would have. So, while a graviton isn't really inevitable, if gravity has a quantum nature, it is by far the odds on favorite explanation for how gravity works.

 

[haushofer]

Summary:: Why are gravitons expected to exist when gravity is just warped space time?

As per the summary I don't understand why physicists talk as if gravitons are inevitable, when gravity is just curved spacetime? Why would curved spacetime have a particle?

It's not that "curved spacetime has a particle". The idea is that curved spacetime, if you'd zoom in enough, is a coherent state of gravitons, like photons make up a laserbeam. It is expected that this idea gives at least a good approximation to a quantum theory of gravity, whatever that is. But just as phonons aren't elementary particles, the graviton description could also be just an effective description at most.

See also Fierz-Pauli theory.

 

[atyy]

I don't think physicists do talk as if gravitons were inevitable. Quantum gravity, yes, but I don't think all current candidates for a theory of quantum gravity include gravitons, although I might be wrong.

Currently, all successful candidates for a theory of quantum gravity are expected to include gravitons. This is because GR can be understood as a low energy effective quantum field theory.

https://arxiv.org/abs/gr-qc/0311082
Quantum Gravity in Everyday Life: General Relativity as an Effective Field Theory
C.P. Burgess

http://www.scholarpedia.org/article/Quantum_gravity_as_a_low_energy_effective_field_theory
Quantum gravity as a low energy effective field theory
John F. Donoghue

 

[ohwilleke]

Currently, all successful candidates for a theory of quantum gravity are expected to include gravitons.

Is this true of quantum gravity theories that quantize space-time along the lines of loop quantum gravity and causal dynamical sets? Or are you presuming that these theory are not successful candidates?

 

[arivero]

I think main reasonment goes as "a two-index tensor is a sum of tensor products of one-index tensor. Under quantization, one-index tensors become spin-1 particles. So two-index tensors become spin-2 particles. And well, the metric field is a two-index tensor." Same arguments with R_mu_nu, etc

Other line, minor, is "forces at tree level are one-boson exchange. If boson spin is odd, equal charges repel. So gravity must be spin zero or spin two" Other considerations discard spin zero, "scalar gravity"

 

[haushofer]

Is this true of quantum gravity theories that quantize space-time along the lines of loop quantum gravity and causal dynamical sets? Or are you presuming that these theory are not successful candidates?

Afaik gravitons (propagators) appear in LQG like phonons in solid state physics; not as fundamental states, but as a low energy limit of the quantized spacetime. The details however (e.g. the background which is used) are not known to me.

 

[Omega0]

Well, I am not even sure if it is important to quantazise the gravitational energy. For me it makes not too much sense to believe that "particles" which come from a linearized theory from a strongly non-linear theory (by definition) in the real world would be the sort of photons we already observe.
I believe that the reality is somehow deeper.

 

[Omega0]

As per the summary I don't understand why physicists talk as if gravitons are inevitable, when gravity is just curved spacetime? Why would curved spacetime have a particle?

To add something to my last post: I would recommend to have an introduction to Solid State Physics. You will be amazed how many particles exist where you might never have heard from. It is a big particle world, even if we never expect to see one of them

 

[atyy]

Is this true of quantum gravity theories that quantize space-time along the lines of loop quantum gravity and causal dynamical sets? Or are you presuming that these theory are not successful candidates?

Yes, I do mean that if LQG and causal sets are successful, then there will be a graviton in those theories. I'm not so sure about specifics in causal sets, but here is an attempt to see if LQG can get the graviton. I'm not sure what the current status of LQG is, but this shows that they do expect a graviton to emerge in some regime if the theory is successful.

https://arxiv.org/abs/gr-qc/0604044
Graviton propagator in loop quantum gravity
Eugenio Bianchi, Leonardo Modesto, Carlo Rovelli, Simone Speziale

Here's a more recent review, where there is also discussion about how to get the correct graviton behavior at low energies.

https://arxiv.org/abs/2104.04394
A Short Review of Loop Quantum Gravity
Abhay Ashtekar, Eugenio Bianchi

Gravitons are also mentioned in a very recent LQG paper.

https://arxiv.org/abs/2005.00883
Manifestly Gauge-Invariant Cosmological Perturbation Theory from Full Loop Quantum Gravity
Muxin Han, Haida Li, Hongguang Liu

 

[f todd baker]

For some time I found myself puzzled by why quantum gravity was being pursued since I subscribed to something like "general relativity is just geometry". What satisfied me in the end was that it has been noted that warped space-time is just one possible interpretation of Einstein's equations. It may also be viewed as a field theory and therefore should be quantizable. None other than Steven Weinberg has adopted this view. A good readable essay may be found at https://www.quantum-field-theory.net/space-time-curvature-relativity/

 

[arivero]

and therefore should be quantizable

I usually wonder if the only classical theories that are well defined are those that are quantizable.

 

[FactChecker]

It seems to me that there must be something more than simply saying it's geometry. How does a mass change the geometry at a distance? And why does the gravitational effect of a moving mass on the geometry spread at the speed of light?

 

[atyy]

For some time I found myself puzzled by why quantum gravity was being pursued since I subscribed to something like "general relativity is just geometry". What satisfied me in the end was that it has been noted that warped space-time is just one possible interpretation of Einstein's equations. It may also be viewed as a field theory and therefore should be quantizable. None other than Steven Weinberg has adopted this view. A good readable essay may be found at https://www.quantum-field-theory.net/space-time-curvature-relativity/

There is interesting history to this idea. After special relativity was understood, people searched for a theory of gravity compatible with special relativity. The first such theories were those of Nordstrom's in 1912-1913, which considered relativistic gravity as a field in flat spacetime. Einstein (who had been pursuing a geometric theory) and Fokker then showed in 1914 that Nordstrom's theory could be rewritten with gravity as curved spacetime. So the field view and the curved spacetime view have been present since the start.

It turns out that Nordstrom's theory, although it is theoretically possible, is not the one nature chose. Einstein worked out general relativity in 1915, and it turns out that that is the theory that matches observation.

 

[PeroK]

It seems to me that there must be something more than simply saying it's geometry. How does a mass change the geometry at a distance? And why does the gravitational effect of a moving mass on the geometry spread at the speed of light?

The "how" in GR is essentially axiomatic, defined by the Einstein Field Equations - which Einstein developed from some general principles and the requirement to approximate Newtonian gravity in the appropriate limit. They can also be derived using the Einstein-Hilbert action. See, for example:

https://en.wikipedia.org/wiki/Einstein–Hilbert_action

 

[FactChecker]

The "how" in GR is essentially axiomatic, defined by the Einstein Field Equations - which Eisntenin developed from some general principles and the requirement to approximate Newtonian gravity in the appropriate limit. They can also de derived using the Einstein-Hilbert action. See, for example:

https://en.wikipedia.org/wiki/Einstein–Hilbert_action

Thanks. I looked at your reference. My brain cell is now recovering from PTSD. I'll take your word for it.

 

[sbrothy]

Summary:: Why are gravitons expected to exist when gravity is just warped space time?

As per the summary I don't understand why physicists talk as if gravitons are inevitable, when gravity is just curved spacetime? Why would curved spacetime have a particle?

I've also been wanting to ask this question, but was afraid of making a fool of myself. I'm glad to see I'm not the only one puzzled by this...

Regards.

 

[haushofer]

For me, this question arised in the context of describing strings in curved spacetime.

 

[Prishon]

This is basically the same as asking why a smooth electromagnetic wave needs photons. On small scales space is not smooth anymore but bumpy, so to speak. Spacetime on a small scale behaves fundamentally diferent from macroscopic classical spacetime. A smooth gravitational wave is thought to exist of zillions metric carrying (or space distortions themselves) carrying gravitons just as a smooth EM wave is composed of zillions of photons.

I see there is a sceptical reaction. I think I know why. Maybe the comparison with an EM wave is not that good (it is though for DW's). The comparison must be made with the smooth electric field surrounding a charge. That is a condensate of virtual photons (fluctuations in the photon field). Likewise, a mass is surroundinde by (or immersed in) curved smooth space. Likewise, this curved space is a condensate of virtual gravitons.

There are not two different mechanisms at work (particle mediated and metric mediated).

 

[weirdoguy]

On small scales space is not smooth anymore but bumpy

We do not know that. Most of quantum gravity models predict that, but it can be that spacetime is perfectly smooth even on such scales.

That is a condensate of virtual photons

No it is not. "Virtual particles" is just a name for internal lines of Feynman diagrams which are usually drawn in the context of perturbative approach to scattering (and other) processes. In the non-perturbative context virtual particles do not appear, so trying to invoke them in every situation is misleading at best, even though a lot of pop-sci authors do that. As always when it comes to virtual particles, I recommend reading these insights articles:
https://www.physicsforums.com/insights/misconceptions-virtual-particles/
https://www.physicsforums.com/insights/physics-virtual-particles/

 

[PeterDonis]

The OP question has been addressed. Thread closed.